Spread 1 cup of marinara across the bottom of a 6-quart slow cooker.
Layer in half the ravioli, half the spinach, half the remaining sauce and 1 cup mozzarella, then repeat with the remaining ravioli, spinach and sauce.
Season the top with the salt and black pepper, cover and cook on low for 2 to 2 1/2 hours.
Check that the ravioli are tender at the center, then sprinkle the remaining mozzarella over the sauce.
Cover for 10 minutes, until the cheese melts, and scoop gently so the ravioli stay intact.